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      <h1>Introducing Flink Streaming</h1>

      <article>
        <p>09 Feb 2015</p>

<p>This post is the first of a series of blog posts on Flink Streaming,
the recent addition to Apache Flink that makes it possible to analyze
continuous data sources in addition to static files. Flink Streaming
uses the pipelined Flink engine to process data streams in real time
and offers a new API including definition of flexible windows.</p>

<p>In this post, we go through an example that uses the Flink Streaming
API to compute statistics on stock market data that arrive
continuously and combine the stock market data with Twitter streams.
See the <a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html">Streaming Programming
Guide</a> for a
detailed presentation of the Streaming API.</p>

<p>First, we read a bunch of stock price streams and combine them into
one stream of market data. We apply several transformations on this
market data stream, like rolling aggregations per stock. Then we emit
price warning alerts when the prices are rapidly changing. Moving 
towards more advanced features, we compute rolling correlations
between the market data streams and a Twitter stream with stock mentions.</p>

<p>For running the example implementation please use the <em>0.9-SNAPSHOT</em> 
version of Flink as a dependency. The full example code base can be 
found <a href="https://github.com/mbalassi/flink/blob/stockprices/flink-staging/flink-streaming/flink-streaming-examples/src/main/scala/org/apache/flink/streaming/scala/examples/windowing/StockPrices.scala">here</a> in Scala and <a href="https://github.com/mbalassi/flink/blob/stockprices/flink-staging/flink-streaming/flink-streaming-examples/src/main/java/org/apache/flink/streaming/examples/windowing/StockPrices.java">here</a> in Java7.</p>

<p><a href="#top"></a></p>

<p><a href="#top">Back to top</a></p>

<h2 id="reading-from-multiple-inputs">Reading from multiple inputs</h2>

<p>First, let us create the stream of stock prices:</p>

<ol>
  <li>Read a socket stream of stock prices</li>
  <li>Parse the text in the stream to create a stream of <code>StockPrice</code> objects</li>
  <li>Add four other sources tagged with the stock symbol.</li>
  <li>Finally, merge the streams to create a unified stream.</li>
</ol>

<p><img alt="Reading from multiple inputs" src="/img/blog/blog_multi_input.png" width="70%" class="img-responsive center-block" /></p>

<div class="codetabs">
  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="k">def</span> <span class="n">main</span><span class="o">(</span><span class="n">args</span><span class="k">:</span> <span class="kt">Array</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span> <span class="o">{</span>

  <span class="k">val</span> <span class="n">env</span> <span class="k">=</span> <span class="nc">StreamExecutionEnvironment</span><span class="o">.</span><span class="n">getExecutionEnvironment</span>

  <span class="c1">//Read from a socket stream at map it to StockPrice objects</span>
  <span class="k">val</span> <span class="n">socketStockStream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">socketTextStream</span><span class="o">(</span><span class="s">&quot;localhost&quot;</span><span class="o">,</span> <span class="mi">9999</span><span class="o">).</span><span class="n">map</span><span class="o">(</span><span class="n">x</span> <span class="k">=&gt;</span> <span class="o">{</span>
    <span class="k">val</span> <span class="n">split</span> <span class="k">=</span> <span class="n">x</span><span class="o">.</span><span class="n">split</span><span class="o">(</span><span class="s">&quot;,&quot;</span><span class="o">)</span>
    <span class="nc">StockPrice</span><span class="o">(</span><span class="n">split</span><span class="o">(</span><span class="mi">0</span><span class="o">),</span> <span class="n">split</span><span class="o">(</span><span class="mi">1</span><span class="o">).</span><span class="n">toDouble</span><span class="o">)</span>
  <span class="o">})</span>

  <span class="c1">//Generate other stock streams</span>
  <span class="k">val</span> <span class="nc">SPX_Stream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">addSource</span><span class="o">(</span><span class="n">generateStock</span><span class="o">(</span><span class="s">&quot;SPX&quot;</span><span class="o">)(</span><span class="mi">10</span><span class="o">)</span> <span class="k">_</span><span class="o">)</span>
  <span class="k">val</span> <span class="nc">FTSE_Stream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">addSource</span><span class="o">(</span><span class="n">generateStock</span><span class="o">(</span><span class="s">&quot;FTSE&quot;</span><span class="o">)(</span><span class="mi">20</span><span class="o">)</span> <span class="k">_</span><span class="o">)</span>
  <span class="k">val</span> <span class="nc">DJI_Stream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">addSource</span><span class="o">(</span><span class="n">generateStock</span><span class="o">(</span><span class="s">&quot;DJI&quot;</span><span class="o">)(</span><span class="mi">30</span><span class="o">)</span> <span class="k">_</span><span class="o">)</span>
  <span class="k">val</span> <span class="nc">BUX_Stream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">addSource</span><span class="o">(</span><span class="n">generateStock</span><span class="o">(</span><span class="s">&quot;BUX&quot;</span><span class="o">)(</span><span class="mi">40</span><span class="o">)</span> <span class="k">_</span><span class="o">)</span>

  <span class="c1">//Merge all stock streams together</span>
  <span class="k">val</span> <span class="n">stockStream</span> <span class="k">=</span> <span class="n">socketStockStream</span><span class="o">.</span><span class="n">merge</span><span class="o">(</span><span class="nc">SPX_Stream</span><span class="o">,</span> <span class="nc">FTSE_Stream</span><span class="o">,</span> 
    <span class="nc">DJI_Stream</span><span class="o">,</span> <span class="nc">BUX_Stream</span><span class="o">)</span>

  <span class="n">stockStream</span><span class="o">.</span><span class="n">print</span><span class="o">()</span>

  <span class="n">env</span><span class="o">.</span><span class="n">execute</span><span class="o">(</span><span class="s">&quot;Stock stream&quot;</span><span class="o">)</span>
<span class="o">}</span></code></pre></div>

  </div>
  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="kd">public</span> <span class="kd">static</span> <span class="kt">void</span> <span class="nf">main</span><span class="o">(</span><span class="n">String</span><span class="o">[]</span> <span class="n">args</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>

    <span class="kd">final</span> <span class="n">StreamExecutionEnvironment</span> <span class="n">env</span> <span class="o">=</span>
        <span class="n">StreamExecutionEnvironment</span><span class="o">.</span><span class="na">getExecutionEnvironment</span><span class="o">();</span>

    <span class="c1">//Read from a socket stream at map it to StockPrice objects</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">socketStockStream</span> <span class="o">=</span> <span class="n">env</span>
            <span class="o">.</span><span class="na">socketTextStream</span><span class="o">(</span><span class="s">&quot;localhost&quot;</span><span class="o">,</span> <span class="mi">9999</span><span class="o">)</span>
            <span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="k">new</span> <span class="n">MapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">StockPrice</span><span class="o">&gt;()</span> <span class="o">{</span>
                <span class="kd">private</span> <span class="n">String</span><span class="o">[]</span> <span class="n">tokens</span><span class="o">;</span>

                <span class="nd">@Override</span>
                <span class="kd">public</span> <span class="n">StockPrice</span> <span class="nf">map</span><span class="o">(</span><span class="n">String</span> <span class="n">value</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
                    <span class="n">tokens</span> <span class="o">=</span> <span class="n">value</span><span class="o">.</span><span class="na">split</span><span class="o">(</span><span class="s">&quot;,&quot;</span><span class="o">);</span>
                    <span class="k">return</span> <span class="k">new</span> <span class="nf">StockPrice</span><span class="o">(</span><span class="n">tokens</span><span class="o">[</span><span class="mi">0</span><span class="o">],</span>
                        <span class="n">Double</span><span class="o">.</span><span class="na">parseDouble</span><span class="o">(</span><span class="n">tokens</span><span class="o">[</span><span class="mi">1</span><span class="o">]));</span>
                <span class="o">}</span>
            <span class="o">});</span>

    <span class="c1">//Generate other stock streams</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">SPX_stream</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">addSource</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockSource</span><span class="o">(</span><span class="s">&quot;SPX&quot;</span><span class="o">,</span> <span class="mi">10</span><span class="o">));</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">FTSE_stream</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">addSource</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockSource</span><span class="o">(</span><span class="s">&quot;FTSE&quot;</span><span class="o">,</span> <span class="mi">20</span><span class="o">));</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">DJI_stream</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">addSource</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockSource</span><span class="o">(</span><span class="s">&quot;DJI&quot;</span><span class="o">,</span> <span class="mi">30</span><span class="o">));</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">BUX_stream</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">addSource</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockSource</span><span class="o">(</span><span class="s">&quot;BUX&quot;</span><span class="o">,</span> <span class="mi">40</span><span class="o">));</span>

    <span class="c1">//Merge all stock streams together</span>
    <span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">stockStream</span> <span class="o">=</span> <span class="n">socketStockStream</span>
        <span class="o">.</span><span class="na">merge</span><span class="o">(</span><span class="n">SPX_stream</span><span class="o">,</span> <span class="n">FTSE_stream</span><span class="o">,</span> <span class="n">DJI_stream</span><span class="o">,</span> <span class="n">BUX_stream</span><span class="o">);</span>

    <span class="n">stockStream</span><span class="o">.</span><span class="na">print</span><span class="o">();</span>

    <span class="n">env</span><span class="o">.</span><span class="na">execute</span><span class="o">(</span><span class="s">&quot;Stock stream&quot;</span><span class="o">);</span></code></pre></div>

  </div>
</div>

<p>See
<a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html#sources">here</a>
on how you can create streaming sources for Flink Streaming
programs. Flink, of course, has support for reading in streams from
<a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html#stream-connectors">external
sources</a>
such as Apache Kafka, Apache Flume, RabbitMQ, and others. For the sake
of this example, the data streams are simply generated using the
<code>generateStock</code> method:</p>

<div class="codetabs">
  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="k">val</span> <span class="n">symbols</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="s">&quot;SPX&quot;</span><span class="o">,</span> <span class="s">&quot;FTSE&quot;</span><span class="o">,</span> <span class="s">&quot;DJI&quot;</span><span class="o">,</span> <span class="s">&quot;DJT&quot;</span><span class="o">,</span> <span class="s">&quot;BUX&quot;</span><span class="o">,</span> <span class="s">&quot;DAX&quot;</span><span class="o">,</span> <span class="s">&quot;GOOG&quot;</span><span class="o">)</span>

<span class="k">case</span> <span class="k">class</span> <span class="nc">StockPrice</span><span class="o">(</span><span class="n">symbol</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">price</span><span class="k">:</span> <span class="kt">Double</span><span class="o">)</span>

<span class="k">def</span> <span class="n">generateStock</span><span class="o">(</span><span class="n">symbol</span><span class="k">:</span> <span class="kt">String</span><span class="o">)(</span><span class="n">sigma</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)(</span><span class="n">out</span><span class="k">:</span> <span class="kt">Collector</span><span class="o">[</span><span class="kt">StockPrice</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
  <span class="k">var</span> <span class="n">price</span> <span class="k">=</span> <span class="mf">1000.</span>
  <span class="k">while</span> <span class="o">(</span><span class="kc">true</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">price</span> <span class="k">=</span> <span class="n">price</span> <span class="o">+</span> <span class="nc">Random</span><span class="o">.</span><span class="n">nextGaussian</span> <span class="o">*</span> <span class="n">sigma</span>
    <span class="n">out</span><span class="o">.</span><span class="n">collect</span><span class="o">(</span><span class="nc">StockPrice</span><span class="o">(</span><span class="n">symbol</span><span class="o">,</span> <span class="n">price</span><span class="o">))</span>
    <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="nc">Random</span><span class="o">.</span><span class="n">nextInt</span><span class="o">(</span><span class="mi">200</span><span class="o">))</span>
  <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>
  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">ArrayList</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">SYMBOLS</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;(</span>
    <span class="n">Arrays</span><span class="o">.</span><span class="na">asList</span><span class="o">(</span><span class="s">&quot;SPX&quot;</span><span class="o">,</span> <span class="s">&quot;FTSE&quot;</span><span class="o">,</span> <span class="s">&quot;DJI&quot;</span><span class="o">,</span> <span class="s">&quot;DJT&quot;</span><span class="o">,</span> <span class="s">&quot;BUX&quot;</span><span class="o">,</span> <span class="s">&quot;DAX&quot;</span><span class="o">,</span> <span class="s">&quot;GOOG&quot;</span><span class="o">));</span>

<span class="kd">public</span> <span class="kd">static</span> <span class="kd">class</span> <span class="nc">StockPrice</span> <span class="kd">implements</span> <span class="n">Serializable</span> <span class="o">{</span>

    <span class="kd">public</span> <span class="n">String</span> <span class="n">symbol</span><span class="o">;</span>
    <span class="kd">public</span> <span class="n">Double</span> <span class="n">price</span><span class="o">;</span>

    <span class="kd">public</span> <span class="nf">StockPrice</span><span class="o">()</span> <span class="o">{</span>
    <span class="o">}</span>

    <span class="kd">public</span> <span class="nf">StockPrice</span><span class="o">(</span><span class="n">String</span> <span class="n">symbol</span><span class="o">,</span> <span class="n">Double</span> <span class="n">price</span><span class="o">)</span> <span class="o">{</span>
        <span class="k">this</span><span class="o">.</span><span class="na">symbol</span> <span class="o">=</span> <span class="n">symbol</span><span class="o">;</span>
        <span class="k">this</span><span class="o">.</span><span class="na">price</span> <span class="o">=</span> <span class="n">price</span><span class="o">;</span>
    <span class="o">}</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="n">String</span> <span class="nf">toString</span><span class="o">()</span> <span class="o">{</span>
        <span class="k">return</span> <span class="s">&quot;StockPrice{&quot;</span> <span class="o">+</span>
                <span class="s">&quot;symbol=&#39;&quot;</span> <span class="o">+</span> <span class="n">symbol</span> <span class="o">+</span> <span class="sc">&#39;\&#39;&#39;</span> <span class="o">+</span>
                <span class="s">&quot;, count=&quot;</span> <span class="o">+</span> <span class="n">price</span> <span class="o">+</span>
                <span class="sc">&#39;}&#39;</span><span class="o">;</span>
    <span class="o">}</span>
<span class="o">}</span>

<span class="kd">public</span> <span class="kd">final</span> <span class="kd">static</span> <span class="kd">class</span> <span class="nc">StockSource</span> <span class="kd">implements</span> <span class="n">SourceFunction</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="o">{</span>

    <span class="kd">private</span> <span class="n">Double</span> <span class="n">price</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">String</span> <span class="n">symbol</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Integer</span> <span class="n">sigma</span><span class="o">;</span>

    <span class="kd">public</span> <span class="nf">StockSource</span><span class="o">(</span><span class="n">String</span> <span class="n">symbol</span><span class="o">,</span> <span class="n">Integer</span> <span class="n">sigma</span><span class="o">)</span> <span class="o">{</span>
        <span class="k">this</span><span class="o">.</span><span class="na">symbol</span> <span class="o">=</span> <span class="n">symbol</span><span class="o">;</span>
        <span class="k">this</span><span class="o">.</span><span class="na">sigma</span> <span class="o">=</span> <span class="n">sigma</span><span class="o">;</span>
    <span class="o">}</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">invoke</span><span class="o">(</span><span class="n">Collector</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">collector</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="n">price</span> <span class="o">=</span> <span class="n">DEFAULT_PRICE</span><span class="o">;</span>
        <span class="n">Random</span> <span class="n">random</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Random</span><span class="o">();</span>

        <span class="k">while</span> <span class="o">(</span><span class="kc">true</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">price</span> <span class="o">=</span> <span class="n">price</span> <span class="o">+</span> <span class="n">random</span><span class="o">.</span><span class="na">nextGaussian</span><span class="o">()</span> <span class="o">*</span> <span class="n">sigma</span><span class="o">;</span>
            <span class="n">collector</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockPrice</span><span class="o">(</span><span class="n">symbol</span><span class="o">,</span> <span class="n">price</span><span class="o">));</span>
            <span class="n">Thread</span><span class="o">.</span><span class="na">sleep</span><span class="o">(</span><span class="n">random</span><span class="o">.</span><span class="na">nextInt</span><span class="o">(</span><span class="mi">200</span><span class="o">));</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>
</div>

<p>To read from the text socket stream please make sure that you have a
socket running. For the sake of the example executing the following
command in a terminal does the job. You can get
<a href="http://netcat.sourceforge.net/">netcat</a> here if it is not available
on your machine.</p>

<div class="highlight"><pre><code>nc -lk 9999
</code></pre></div>

<p>If we execute the program from our IDE we see the system the
stock prices being generated:</p>

<div class="highlight"><pre><code>INFO    Job execution switched to status RUNNING.
INFO    Socket Stream(1/1) switched to SCHEDULED 
INFO    Socket Stream(1/1) switched to DEPLOYING
INFO    Custom Source(1/1) switched to SCHEDULED 
INFO    Custom Source(1/1) switched to DEPLOYING
…
1&gt; StockPrice{symbol='SPX', count=1011.3405732645239}
2&gt; StockPrice{symbol='SPX', count=1018.3381290039248}
1&gt; StockPrice{symbol='DJI', count=1036.7454894073978}
3&gt; StockPrice{symbol='DJI', count=1135.1170217478427}
3&gt; StockPrice{symbol='BUX', count=1053.667523187687}
4&gt; StockPrice{symbol='BUX', count=1036.552601487263}
</code></pre></div>

<p><a href="#top">Back to top</a></p>

<h2 id="window-aggregations">Window aggregations</h2>

<p>We first compute aggregations on time-based windows of the
data. Flink provides <a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html#window-operators">flexible windowing semantics</a> where windows can
also be defined based on count of records or any custom user defined
logic.</p>

<p>We partition our stream into windows of 10 seconds and slide the
window every 5 seconds. We compute three statistics every 5 seconds.
The first is the minimum price of all stocks, the second produces
maximum price per stock, and the third is the mean stock price 
(using a map window function). Aggregations and groupings can be
performed on named fields of POJOs, making the code more readable.</p>

<p><img alt="Basic windowing aggregations" src="/img/blog/blog_basic_window.png" width="70%" class="img-responsive center-block" /></p>

<div class="codetabs">

  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="c1">//Define the desired time window</span>
<span class="k">val</span> <span class="n">windowedStream</span> <span class="k">=</span> <span class="n">stockStream</span>
  <span class="o">.</span><span class="n">window</span><span class="o">(</span><span class="nc">Time</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mi">10</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">)).</span><span class="n">every</span><span class="o">(</span><span class="nc">Time</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mi">5</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">))</span>

<span class="c1">//Compute some simple statistics on a rolling window</span>
<span class="k">val</span> <span class="n">lowest</span> <span class="k">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="n">minBy</span><span class="o">(</span><span class="s">&quot;price&quot;</span><span class="o">)</span>
<span class="k">val</span> <span class="n">maxByStock</span> <span class="k">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">).</span><span class="n">maxBy</span><span class="o">(</span><span class="s">&quot;price&quot;</span><span class="o">)</span>
<span class="k">val</span> <span class="n">rollingMean</span> <span class="k">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">).</span><span class="n">mapWindow</span><span class="o">(</span><span class="n">mean</span> <span class="k">_</span><span class="o">)</span>

<span class="c1">//Compute the mean of a window</span>
<span class="k">def</span> <span class="n">mean</span><span class="o">(</span><span class="n">ts</span><span class="k">:</span> <span class="kt">Iterable</span><span class="o">[</span><span class="kt">StockPrice</span><span class="o">],</span> <span class="n">out</span><span class="k">:</span> <span class="kt">Collector</span><span class="o">[</span><span class="kt">StockPrice</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
  <span class="k">if</span> <span class="o">(</span><span class="n">ts</span><span class="o">.</span><span class="n">nonEmpty</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">out</span><span class="o">.</span><span class="n">collect</span><span class="o">(</span><span class="nc">StockPrice</span><span class="o">(</span><span class="n">ts</span><span class="o">.</span><span class="n">head</span><span class="o">.</span><span class="n">symbol</span><span class="o">,</span> <span class="n">ts</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="mi">0</span><span class="k">:</span> <span class="kt">Double</span><span class="o">)(</span><span class="k">_</span> <span class="o">+</span> <span class="k">_</span><span class="o">.</span><span class="n">price</span><span class="o">)</span> <span class="o">/</span> <span class="n">ts</span><span class="o">.</span><span class="n">size</span><span class="o">))</span>
  <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="c1">//Define the desired time window</span>
<span class="n">WindowedDataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">windowedStream</span> <span class="o">=</span> <span class="n">stockStream</span>
    <span class="o">.</span><span class="na">window</span><span class="o">(</span><span class="n">Time</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mi">10</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">))</span>
    <span class="o">.</span><span class="na">every</span><span class="o">(</span><span class="n">Time</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mi">5</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">));</span>

<span class="c1">//Compute some simple statistics on a rolling window</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">lowest</span> <span class="o">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="na">minBy</span><span class="o">(</span><span class="s">&quot;price&quot;</span><span class="o">).</span><span class="na">flatten</span><span class="o">();</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">maxByStock</span> <span class="o">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="na">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
    <span class="o">.</span><span class="na">maxBy</span><span class="o">(</span><span class="s">&quot;price&quot;</span><span class="o">).</span><span class="na">flatten</span><span class="o">();</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">rollingMean</span> <span class="o">=</span> <span class="n">windowedStream</span><span class="o">.</span><span class="na">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
    <span class="o">.</span><span class="na">mapWindow</span><span class="o">(</span><span class="k">new</span> <span class="nf">WindowMean</span><span class="o">()).</span><span class="na">flatten</span><span class="o">();</span>

<span class="c1">//Compute the mean of a window</span>
<span class="kd">public</span> <span class="kd">final</span> <span class="kd">static</span> <span class="kd">class</span> <span class="nc">WindowMean</span> <span class="kd">implements</span> 
    <span class="n">WindowMapFunction</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">,</span> <span class="n">StockPrice</span><span class="o">&gt;</span> <span class="o">{</span>

    <span class="kd">private</span> <span class="n">Double</span> <span class="n">sum</span> <span class="o">=</span> <span class="mf">0.0</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Integer</span> <span class="n">count</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">String</span> <span class="n">symbol</span> <span class="o">=</span> <span class="s">&quot;&quot;</span><span class="o">;</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">mapWindow</span><span class="o">(</span><span class="n">Iterable</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">values</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">out</span><span class="o">)</span> 
        <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>

        <span class="k">if</span> <span class="o">(</span><span class="n">values</span><span class="o">.</span><span class="na">iterator</span><span class="o">().</span><span class="na">hasNext</span><span class="o">())</span> <span class="o">{</span><span class="n">s</span>
            <span class="nf">for</span> <span class="o">(</span><span class="n">StockPrice</span> <span class="n">sp</span> <span class="o">:</span> <span class="n">values</span><span class="o">)</span> <span class="o">{</span>
                <span class="n">sum</span> <span class="o">+=</span> <span class="n">sp</span><span class="o">.</span><span class="na">price</span><span class="o">;</span>
                <span class="n">symbol</span> <span class="o">=</span> <span class="n">sp</span><span class="o">.</span><span class="na">symbol</span><span class="o">;</span>
                <span class="n">count</span><span class="o">++;</span>
            <span class="o">}</span>
            <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="k">new</span> <span class="nf">StockPrice</span><span class="o">(</span><span class="n">symbol</span><span class="o">,</span> <span class="n">sum</span> <span class="o">/</span> <span class="n">count</span><span class="o">));</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

</div>

<p>Let us note that to print a windowed stream one has to flatten it first,
thus getting rid of the windowing logic. For example execute 
<code>maxByStock.flatten().print()</code> to print the stream of maximum prices of
 the time windows by stock. For Scala <code>flatten()</code> is called implicitly
when needed.</p>

<p><a href="#top">Back to top</a></p>

<h2 id="data-driven-windows">Data-driven windows</h2>

<p>The most interesting event in the stream is when the price of a stock
is changing rapidly. We can send a warning when a stock price changes
more than 5% since the last warning. To do that, we use a delta-based window providing a
threshold on when the computation will be triggered, a function to
compute the difference and a default value with which the first record
is compared. We also create a <code>Count</code> data type to count the warnings
every 30 seconds.</p>

<p><img alt="Data-driven windowing semantics" src="/img/blog/blog_data_driven.png" width="100%" class="img-responsive center-block" /></p>

<div class="codetabs">

  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="k">case</span> <span class="k">class</span> <span class="nc">Count</span><span class="o">(</span><span class="n">symbol</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">count</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="k">val</span> <span class="n">defaultPrice</span> <span class="k">=</span> <span class="nc">StockPrice</span><span class="o">(</span><span class="s">&quot;&quot;</span><span class="o">,</span> <span class="mi">1000</span><span class="o">)</span>

<span class="c1">//Use delta policy to create price change warnings</span>
<span class="k">val</span> <span class="n">priceWarnings</span> <span class="k">=</span> <span class="n">stockStream</span><span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
  <span class="o">.</span><span class="n">window</span><span class="o">(</span><span class="nc">Delta</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mf">0.05</span><span class="o">,</span> <span class="n">priceChange</span><span class="o">,</span> <span class="n">defaultPrice</span><span class="o">))</span>
  <span class="o">.</span><span class="n">mapWindow</span><span class="o">(</span><span class="n">sendWarning</span> <span class="k">_</span><span class="o">)</span>

<span class="c1">//Count the number of warnings every half a minute</span>
<span class="k">val</span> <span class="n">warningsPerStock</span> <span class="k">=</span> <span class="n">priceWarnings</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="k">_</span><span class="o">,</span> <span class="mi">1</span><span class="o">))</span>
  <span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
  <span class="o">.</span><span class="n">window</span><span class="o">(</span><span class="nc">Time</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">))</span>
  <span class="o">.</span><span class="n">sum</span><span class="o">(</span><span class="s">&quot;count&quot;</span><span class="o">)</span>

<span class="k">def</span> <span class="n">priceChange</span><span class="o">(</span><span class="n">p1</span><span class="k">:</span> <span class="kt">StockPrice</span><span class="o">,</span> <span class="n">p2</span><span class="k">:</span> <span class="kt">StockPrice</span><span class="o">)</span><span class="k">:</span> <span class="kt">Double</span> <span class="o">=</span> <span class="o">{</span>
  <span class="nc">Math</span><span class="o">.</span><span class="n">abs</span><span class="o">(</span><span class="n">p1</span><span class="o">.</span><span class="n">price</span> <span class="o">/</span> <span class="n">p2</span><span class="o">.</span><span class="n">price</span> <span class="o">-</span> <span class="mi">1</span><span class="o">)</span>
<span class="o">}</span>

<span class="k">def</span> <span class="n">sendWarning</span><span class="o">(</span><span class="n">ts</span><span class="k">:</span> <span class="kt">Iterable</span><span class="o">[</span><span class="kt">StockPrice</span><span class="o">],</span> <span class="n">out</span><span class="k">:</span> <span class="kt">Collector</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
  <span class="k">if</span> <span class="o">(</span><span class="n">ts</span><span class="o">.</span><span class="n">nonEmpty</span><span class="o">)</span> <span class="n">out</span><span class="o">.</span><span class="n">collect</span><span class="o">(</span><span class="n">ts</span><span class="o">.</span><span class="n">head</span><span class="o">.</span><span class="n">symbol</span><span class="o">)</span>
<span class="o">}</span></code></pre></div>

  </div>

  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">Double</span> <span class="n">DEFAULT_PRICE</span> <span class="o">=</span> <span class="mi">1000</span><span class="o">.;</span>
<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">StockPrice</span> <span class="n">DEFAULT_STOCK_PRICE</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">StockPrice</span><span class="o">(</span><span class="s">&quot;&quot;</span><span class="o">,</span> <span class="n">DEFAULT_PRICE</span><span class="o">);</span>

<span class="c1">//Use delta policy to create price change warnings</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">priceWarnings</span> <span class="o">=</span> <span class="n">stockStream</span><span class="o">.</span><span class="na">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
    <span class="o">.</span><span class="na">window</span><span class="o">(</span><span class="n">Delta</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mf">0.05</span><span class="o">,</span> <span class="k">new</span> <span class="n">DeltaFunction</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;()</span> <span class="o">{</span>
        <span class="nd">@Override</span>
        <span class="kd">public</span> <span class="kt">double</span> <span class="nf">getDelta</span><span class="o">(</span><span class="n">StockPrice</span> <span class="n">oldDataPoint</span><span class="o">,</span> <span class="n">StockPrice</span> <span class="n">newDataPoint</span><span class="o">)</span> <span class="o">{</span>
            <span class="k">return</span> <span class="n">Math</span><span class="o">.</span><span class="na">abs</span><span class="o">(</span><span class="n">oldDataPoint</span><span class="o">.</span><span class="na">price</span> <span class="o">-</span> <span class="n">newDataPoint</span><span class="o">.</span><span class="na">price</span><span class="o">);</span>
        <span class="o">}</span>
    <span class="o">},</span> <span class="n">DEFAULT_STOCK_PRICE</span><span class="o">))</span>
<span class="o">.</span><span class="na">mapWindow</span><span class="o">(</span><span class="k">new</span> <span class="nf">SendWarning</span><span class="o">()).</span><span class="na">flatten</span><span class="o">();</span>

<span class="c1">//Count the number of warnings every half a minute</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">&gt;</span> <span class="n">warningsPerStock</span> <span class="o">=</span> <span class="n">priceWarnings</span><span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="k">new</span> <span class="n">MapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">Count</span><span class="o">&gt;()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="n">Count</span> <span class="nf">map</span><span class="o">(</span><span class="n">String</span> <span class="n">value</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="k">return</span> <span class="k">new</span> <span class="nf">Count</span><span class="o">(</span><span class="n">value</span><span class="o">,</span> <span class="mi">1</span><span class="o">);</span>
    <span class="o">}</span>
<span class="o">}).</span><span class="na">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">).</span><span class="na">window</span><span class="o">(</span><span class="n">Time</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)).</span><span class="na">sum</span><span class="o">(</span><span class="s">&quot;count&quot;</span><span class="o">).</span><span class="na">flatten</span><span class="o">();</span>

<span class="kd">public</span> <span class="kd">static</span> <span class="kd">class</span> <span class="nc">Count</span> <span class="kd">implements</span> <span class="n">Serializable</span> <span class="o">{</span>
    <span class="kd">public</span> <span class="n">String</span> <span class="n">symbol</span><span class="o">;</span>
    <span class="kd">public</span> <span class="n">Integer</span> <span class="n">count</span><span class="o">;</span>

    <span class="kd">public</span> <span class="nf">Count</span><span class="o">()</span> <span class="o">{</span>
    <span class="o">}</span>

    <span class="kd">public</span> <span class="nf">Count</span><span class="o">(</span><span class="n">String</span> <span class="n">symbol</span><span class="o">,</span> <span class="n">Integer</span> <span class="n">count</span><span class="o">)</span> <span class="o">{</span>
        <span class="k">this</span><span class="o">.</span><span class="na">symbol</span> <span class="o">=</span> <span class="n">symbol</span><span class="o">;</span>
        <span class="k">this</span><span class="o">.</span><span class="na">count</span> <span class="o">=</span> <span class="n">count</span><span class="o">;</span>
    <span class="o">}</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="n">String</span> <span class="nf">toString</span><span class="o">()</span> <span class="o">{</span>
        <span class="k">return</span> <span class="s">&quot;Count{&quot;</span> <span class="o">+</span>
                <span class="s">&quot;symbol=&#39;&quot;</span> <span class="o">+</span> <span class="n">symbol</span> <span class="o">+</span> <span class="sc">&#39;\&#39;&#39;</span> <span class="o">+</span>
                <span class="s">&quot;, count=&quot;</span> <span class="o">+</span> <span class="n">count</span> <span class="o">+</span>
                <span class="sc">&#39;}&#39;</span><span class="o">;</span>
    <span class="o">}</span>
<span class="o">}</span>

<span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">SendWarning</span> <span class="kd">implements</span> <span class="n">MapWindowFunction</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">mapWindow</span><span class="o">(</span><span class="n">Iterable</span><span class="o">&lt;</span><span class="n">StockPrice</span><span class="o">&gt;</span> <span class="n">values</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">out</span><span class="o">)</span> 
        <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>

        <span class="k">if</span> <span class="o">(</span><span class="n">values</span><span class="o">.</span><span class="na">iterator</span><span class="o">().</span><span class="na">hasNext</span><span class="o">())</span> <span class="o">{</span>
            <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">values</span><span class="o">.</span><span class="na">iterator</span><span class="o">().</span><span class="na">next</span><span class="o">().</span><span class="na">symbol</span><span class="o">);</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

</div>

<p><a href="#top">Back to top</a></p>

<h2 id="combining-with-a-twitter-stream">Combining with a Twitter stream</h2>

<p>Next, we will read a Twitter stream and correlate it with our stock
price stream. Flink has support for connecting to <a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html#twitter-streaming-api">Twitter’s
API</a>,
but for the sake of this example we generate dummy tweet data.</p>

<p><img alt="Social media analytics" src="/img/blog/blog_social_media.png" width="100%" class="img-responsive center-block" /></p>

<div class="codetabs">

  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="c1">//Read a stream of tweets</span>
<span class="k">val</span> <span class="n">tweetStream</span> <span class="k">=</span> <span class="n">env</span><span class="o">.</span><span class="n">addSource</span><span class="o">(</span><span class="n">generateTweets</span> <span class="k">_</span><span class="o">)</span>

<span class="c1">//Extract the stock symbols</span>
<span class="k">val</span> <span class="n">mentionedSymbols</span> <span class="k">=</span> <span class="n">tweetStream</span><span class="o">.</span><span class="n">flatMap</span><span class="o">(</span><span class="n">tweet</span> <span class="k">=&gt;</span> <span class="n">tweet</span><span class="o">.</span><span class="n">split</span><span class="o">(</span><span class="s">&quot; &quot;</span><span class="o">))</span>
  <span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">toUpperCase</span><span class="o">())</span>
  <span class="o">.</span><span class="n">filter</span><span class="o">(</span><span class="n">symbols</span><span class="o">.</span><span class="n">contains</span><span class="o">(</span><span class="k">_</span><span class="o">))</span>

<span class="c1">//Count the extracted symbols</span>
<span class="k">val</span> <span class="n">tweetsPerStock</span> <span class="k">=</span> <span class="n">mentionedSymbols</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="k">_</span><span class="o">,</span> <span class="mi">1</span><span class="o">))</span>
  <span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
  <span class="o">.</span><span class="n">window</span><span class="o">(</span><span class="nc">Time</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">))</span>
  <span class="o">.</span><span class="n">sum</span><span class="o">(</span><span class="s">&quot;count&quot;</span><span class="o">)</span>

<span class="k">def</span> <span class="n">generateTweets</span><span class="o">(</span><span class="n">out</span><span class="k">:</span> <span class="kt">Collector</span><span class="o">[</span><span class="kt">String</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
  <span class="k">while</span> <span class="o">(</span><span class="kc">true</span><span class="o">)</span> <span class="o">{</span>
    <span class="k">val</span> <span class="n">s</span> <span class="k">=</span> <span class="k">for</span> <span class="o">(</span><span class="n">i</span> <span class="k">&lt;-</span> <span class="mi">1</span> <span class="n">to</span> <span class="mi">3</span><span class="o">)</span> <span class="k">yield</span> <span class="o">(</span><span class="n">symbols</span><span class="o">(</span><span class="nc">Random</span><span class="o">.</span><span class="n">nextInt</span><span class="o">(</span><span class="n">symbols</span><span class="o">.</span><span class="n">size</span><span class="o">)))</span>
    <span class="n">out</span><span class="o">.</span><span class="n">collect</span><span class="o">(</span><span class="n">s</span><span class="o">.</span><span class="n">mkString</span><span class="o">(</span><span class="s">&quot; &quot;</span><span class="o">))</span>
    <span class="nc">Thread</span><span class="o">.</span><span class="n">sleep</span><span class="o">(</span><span class="nc">Random</span><span class="o">.</span><span class="n">nextInt</span><span class="o">(</span><span class="mi">500</span><span class="o">))</span>
  <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="c1">//Read a stream of tweets</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">tweetStream</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="na">addSource</span><span class="o">(</span><span class="k">new</span> <span class="nf">TweetSource</span><span class="o">());</span>

<span class="c1">//Extract the stock symbols</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">mentionedSymbols</span> <span class="o">=</span> <span class="n">tweetStream</span><span class="o">.</span><span class="na">flatMap</span><span class="o">(</span>
    <span class="k">new</span> <span class="n">FlatMapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">String</span><span class="o">&gt;()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">flatMap</span><span class="o">(</span><span class="n">String</span> <span class="n">value</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">out</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="n">String</span><span class="o">[]</span> <span class="n">words</span> <span class="o">=</span> <span class="n">value</span><span class="o">.</span><span class="na">split</span><span class="o">(</span><span class="s">&quot; &quot;</span><span class="o">);</span>
        <span class="k">for</span> <span class="o">(</span><span class="n">String</span> <span class="n">word</span> <span class="o">:</span> <span class="n">words</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">word</span><span class="o">.</span><span class="na">toUpperCase</span><span class="o">());</span>
        <span class="o">}</span>
    <span class="o">}</span>
<span class="o">}).</span><span class="na">filter</span><span class="o">(</span><span class="k">new</span> <span class="n">FilterFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">boolean</span> <span class="nf">filter</span><span class="o">(</span><span class="n">String</span> <span class="n">value</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="k">return</span> <span class="n">SYMBOLS</span><span class="o">.</span><span class="na">contains</span><span class="o">(</span><span class="n">value</span><span class="o">);</span>
    <span class="o">}</span>
<span class="o">});</span>

<span class="c1">//Count the extracted symbols</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">&gt;</span> <span class="n">tweetsPerStock</span> <span class="o">=</span> <span class="n">mentionedSymbols</span><span class="o">.</span><span class="na">map</span><span class="o">(</span><span class="k">new</span> <span class="n">MapFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">,</span> <span class="n">Count</span><span class="o">&gt;()</span> <span class="o">{</span>
    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="n">Count</span> <span class="nf">map</span><span class="o">(</span><span class="n">String</span> <span class="n">value</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="k">return</span> <span class="k">new</span> <span class="nf">Count</span><span class="o">(</span><span class="n">value</span><span class="o">,</span> <span class="mi">1</span><span class="o">);</span>
    <span class="o">}</span>
<span class="o">}).</span><span class="na">groupBy</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">).</span><span class="na">window</span><span class="o">(</span><span class="n">Time</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)).</span><span class="na">sum</span><span class="o">(</span><span class="s">&quot;count&quot;</span><span class="o">).</span><span class="na">flatten</span><span class="o">();</span>

<span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">TweetSource</span> <span class="kd">implements</span> <span class="n">SourceFunction</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="o">{</span>
    <span class="n">Random</span> <span class="n">random</span><span class="o">;</span>
    <span class="n">StringBuilder</span> <span class="n">stringBuilder</span><span class="o">;</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">invoke</span><span class="o">(</span><span class="n">Collector</span><span class="o">&lt;</span><span class="n">String</span><span class="o">&gt;</span> <span class="n">collector</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
        <span class="n">random</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">Random</span><span class="o">();</span>
        <span class="n">stringBuilder</span> <span class="o">=</span> <span class="k">new</span> <span class="nf">StringBuilder</span><span class="o">();</span>

        <span class="k">while</span> <span class="o">(</span><span class="kc">true</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">stringBuilder</span><span class="o">.</span><span class="na">setLength</span><span class="o">(</span><span class="mi">0</span><span class="o">);</span>
            <span class="k">for</span> <span class="o">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="mi">3</span><span class="o">;</span> <span class="n">i</span><span class="o">++)</span> <span class="o">{</span>
                <span class="n">stringBuilder</span><span class="o">.</span><span class="na">append</span><span class="o">(</span><span class="s">&quot; &quot;</span><span class="o">);</span>
                <span class="n">stringBuilder</span><span class="o">.</span><span class="na">append</span><span class="o">(</span><span class="n">SYMBOLS</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">random</span><span class="o">.</span><span class="na">nextInt</span><span class="o">(</span><span class="n">SYMBOLS</span><span class="o">.</span><span class="na">size</span><span class="o">())));</span>
            <span class="o">}</span>
            <span class="n">collector</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">stringBuilder</span><span class="o">.</span><span class="na">toString</span><span class="o">());</span>
            <span class="n">Thread</span><span class="o">.</span><span class="na">sleep</span><span class="o">(</span><span class="mi">500</span><span class="o">);</span>
        <span class="o">}</span>

    <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

</div>

<p><a href="#top">Back to top</a></p>

<h2 id="streaming-joins">Streaming joins</h2>

<p>Finally, we join real-time tweets and stock prices and compute a
rolling correlation between the number of price warnings and the
number of mentions of a given stock in the Twitter stream. As both of
these data streams are potentially infinite, we apply the join on a
30-second window.</p>

<p><img alt="Streaming joins" src="/img/blog/blog_stream_join.png" width="60%" class="img-responsive center-block" /></p>

<div class="codetabs">

  <div data-lang="scala">

    <div class="highlight"><pre><code class="language-scala" data-lang="scala"><span class="c1">//Join warnings and parsed tweets</span>
<span class="k">val</span> <span class="n">tweetsAndWarning</span> <span class="k">=</span> <span class="n">warningsPerStock</span><span class="o">.</span><span class="n">join</span><span class="o">(</span><span class="n">tweetsPerStock</span><span class="o">)</span>
  <span class="o">.</span><span class="n">onWindow</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">)</span>
  <span class="o">.</span><span class="n">where</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
  <span class="o">.</span><span class="n">equalTo</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span> <span class="o">{</span> <span class="o">(</span><span class="n">c1</span><span class="o">,</span> <span class="n">c2</span><span class="o">)</span> <span class="k">=&gt;</span> <span class="o">(</span><span class="n">c1</span><span class="o">.</span><span class="n">count</span><span class="o">,</span> <span class="n">c2</span><span class="o">.</span><span class="n">count</span><span class="o">)</span> <span class="o">}</span>

<span class="k">val</span> <span class="n">rollingCorrelation</span> <span class="k">=</span> <span class="n">tweetsAndWarning</span><span class="o">.</span><span class="n">window</span><span class="o">(</span><span class="nc">Time</span><span class="o">.</span><span class="n">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="nc">SECONDS</span><span class="o">))</span>
  <span class="o">.</span><span class="n">mapWindow</span><span class="o">(</span><span class="n">computeCorrelation</span> <span class="k">_</span><span class="o">)</span>

<span class="n">rollingCorrelation</span> <span class="n">print</span>

<span class="c1">//Compute rolling correlation</span>
<span class="k">def</span> <span class="n">computeCorrelation</span><span class="o">(</span><span class="n">input</span><span class="k">:</span> <span class="kt">Iterable</span><span class="o">[(</span><span class="kt">Int</span>, <span class="kt">Int</span><span class="o">)],</span> <span class="n">out</span><span class="k">:</span> <span class="kt">Collector</span><span class="o">[</span><span class="kt">Double</span><span class="o">])</span> <span class="k">=</span> <span class="o">{</span>
  <span class="k">if</span> <span class="o">(</span><span class="n">input</span><span class="o">.</span><span class="n">nonEmpty</span><span class="o">)</span> <span class="o">{</span>
    <span class="k">val</span> <span class="n">var1</span> <span class="k">=</span> <span class="n">input</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">_1</span><span class="o">)</span>
    <span class="k">val</span> <span class="n">mean1</span> <span class="k">=</span> <span class="n">average</span><span class="o">(</span><span class="n">var1</span><span class="o">)</span>
    <span class="k">val</span> <span class="n">var2</span> <span class="k">=</span> <span class="n">input</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">_2</span><span class="o">)</span>
    <span class="k">val</span> <span class="n">mean2</span> <span class="k">=</span> <span class="n">average</span><span class="o">(</span><span class="n">var2</span><span class="o">)</span>

    <span class="k">val</span> <span class="n">cov</span> <span class="k">=</span> <span class="n">average</span><span class="o">(</span><span class="n">var1</span><span class="o">.</span><span class="n">zip</span><span class="o">(</span><span class="n">var2</span><span class="o">).</span><span class="n">map</span><span class="o">(</span><span class="n">xy</span> <span class="k">=&gt;</span> <span class="o">(</span><span class="n">xy</span><span class="o">.</span><span class="n">_1</span> <span class="o">-</span> <span class="n">mean1</span><span class="o">)</span> <span class="o">*</span> <span class="o">(</span><span class="n">xy</span><span class="o">.</span><span class="n">_2</span> <span class="o">-</span> <span class="n">mean2</span><span class="o">)))</span>
    <span class="k">val</span> <span class="n">d1</span> <span class="k">=</span> <span class="nc">Math</span><span class="o">.</span><span class="n">sqrt</span><span class="o">(</span><span class="n">average</span><span class="o">(</span><span class="n">var1</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">x</span> <span class="k">=&gt;</span> <span class="nc">Math</span><span class="o">.</span><span class="n">pow</span><span class="o">((</span><span class="n">x</span> <span class="o">-</span> <span class="n">mean1</span><span class="o">),</span> <span class="mi">2</span><span class="o">))))</span>
    <span class="k">val</span> <span class="n">d2</span> <span class="k">=</span> <span class="nc">Math</span><span class="o">.</span><span class="n">sqrt</span><span class="o">(</span><span class="n">average</span><span class="o">(</span><span class="n">var2</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">x</span> <span class="k">=&gt;</span> <span class="nc">Math</span><span class="o">.</span><span class="n">pow</span><span class="o">((</span><span class="n">x</span> <span class="o">-</span> <span class="n">mean2</span><span class="o">),</span> <span class="mi">2</span><span class="o">))))</span>

    <span class="n">out</span><span class="o">.</span><span class="n">collect</span><span class="o">(</span><span class="n">cov</span> <span class="o">/</span> <span class="o">(</span><span class="n">d1</span> <span class="o">*</span> <span class="n">d2</span><span class="o">))</span>
  <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

  <div data-lang="java7">

    <div class="highlight"><pre><code class="language-java" data-lang="java"><span class="c1">//Join warnings and parsed tweets</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;&gt;</span> <span class="n">tweetsAndWarning</span> <span class="o">=</span> <span class="n">warningsPerStock</span>
    <span class="o">.</span><span class="na">join</span><span class="o">(</span><span class="n">tweetsPerStock</span><span class="o">)</span>
    <span class="o">.</span><span class="na">onWindow</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">)</span>
    <span class="o">.</span><span class="na">where</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
    <span class="o">.</span><span class="na">equalTo</span><span class="o">(</span><span class="s">&quot;symbol&quot;</span><span class="o">)</span>
    <span class="o">.</span><span class="na">with</span><span class="o">(</span><span class="k">new</span> <span class="n">JoinFunction</span><span class="o">&lt;</span><span class="n">Count</span><span class="o">,</span> <span class="n">Count</span><span class="o">,</span> <span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;&gt;()</span> <span class="o">{</span>
        <span class="nd">@Override</span>
        <span class="kd">public</span> <span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;</span> <span class="nf">join</span><span class="o">(</span><span class="n">Count</span> <span class="n">first</span><span class="o">,</span> <span class="n">Count</span> <span class="n">second</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>
            <span class="k">return</span> <span class="k">new</span> <span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;(</span><span class="n">first</span><span class="o">.</span><span class="na">count</span><span class="o">,</span> <span class="n">second</span><span class="o">.</span><span class="na">count</span><span class="o">);</span>
            <span class="o">}</span>
    <span class="o">});</span>

<span class="c1">//Compute rolling correlation</span>
<span class="n">DataStream</span><span class="o">&lt;</span><span class="n">Double</span><span class="o">&gt;</span> <span class="n">rollingCorrelation</span> <span class="o">=</span> <span class="n">tweetsAndWarning</span>
    <span class="o">.</span><span class="na">window</span><span class="o">(</span><span class="n">Time</span><span class="o">.</span><span class="na">of</span><span class="o">(</span><span class="mi">30</span><span class="o">,</span> <span class="n">TimeUnit</span><span class="o">.</span><span class="na">SECONDS</span><span class="o">))</span>
    <span class="o">.</span><span class="na">mapWindow</span><span class="o">(</span><span class="k">new</span> <span class="nf">WindowCorrelation</span><span class="o">());</span>

<span class="n">rollingCorrelation</span><span class="o">.</span><span class="na">print</span><span class="o">();</span>

<span class="kd">public</span> <span class="kd">static</span> <span class="kd">final</span> <span class="kd">class</span> <span class="nc">WindowCorrelation</span>
    <span class="kd">implements</span> <span class="n">WindowMapFunction</span><span class="o">&lt;</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;,</span> <span class="n">Double</span><span class="o">&gt;</span> <span class="o">{</span>

    <span class="kd">private</span> <span class="n">Integer</span> <span class="n">leftSum</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Integer</span> <span class="n">rightSum</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Integer</span> <span class="n">count</span><span class="o">;</span>

    <span class="kd">private</span> <span class="n">Double</span> <span class="n">leftMean</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Double</span> <span class="n">rightMean</span><span class="o">;</span>

    <span class="kd">private</span> <span class="n">Double</span> <span class="n">cov</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Double</span> <span class="n">leftSd</span><span class="o">;</span>
    <span class="kd">private</span> <span class="n">Double</span> <span class="n">rightSd</span><span class="o">;</span>

    <span class="nd">@Override</span>
    <span class="kd">public</span> <span class="kt">void</span> <span class="nf">mapWindow</span><span class="o">(</span><span class="n">Iterable</span><span class="o">&lt;</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;&gt;</span> <span class="n">values</span><span class="o">,</span> <span class="n">Collector</span><span class="o">&lt;</span><span class="n">Double</span><span class="o">&gt;</span> <span class="n">out</span><span class="o">)</span> 
        <span class="kd">throws</span> <span class="n">Exception</span> <span class="o">{</span>

        <span class="n">leftSum</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span>
        <span class="n">rightSum</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span>
        <span class="n">count</span> <span class="o">=</span> <span class="mi">0</span><span class="o">;</span>

        <span class="n">cov</span> <span class="o">=</span> <span class="mi">0</span><span class="o">.;</span>
        <span class="n">leftSd</span> <span class="o">=</span> <span class="mi">0</span><span class="o">.;</span>
        <span class="n">rightSd</span> <span class="o">=</span> <span class="mi">0</span><span class="o">.;</span>

        <span class="c1">//compute mean for both sides, save count</span>
        <span class="k">for</span> <span class="o">(</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;</span> <span class="n">pair</span> <span class="o">:</span> <span class="n">values</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">leftSum</span> <span class="o">+=</span> <span class="n">pair</span><span class="o">.</span><span class="na">f0</span><span class="o">;</span>
            <span class="n">rightSum</span> <span class="o">+=</span> <span class="n">pair</span><span class="o">.</span><span class="na">f1</span><span class="o">;</span>
            <span class="n">count</span><span class="o">++;</span>
        <span class="o">}</span>

        <span class="n">leftMean</span> <span class="o">=</span> <span class="n">leftSum</span><span class="o">.</span><span class="na">doubleValue</span><span class="o">()</span> <span class="o">/</span> <span class="n">count</span><span class="o">;</span>
        <span class="n">rightMean</span> <span class="o">=</span> <span class="n">rightSum</span><span class="o">.</span><span class="na">doubleValue</span><span class="o">()</span> <span class="o">/</span> <span class="n">count</span><span class="o">;</span>

        <span class="c1">//compute covariance &amp; std. deviations</span>
        <span class="k">for</span> <span class="o">(</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;</span> <span class="n">pair</span> <span class="o">:</span> <span class="n">values</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">cov</span> <span class="o">+=</span> <span class="o">(</span><span class="n">pair</span><span class="o">.</span><span class="na">f0</span> <span class="o">-</span> <span class="n">leftMean</span><span class="o">)</span> <span class="o">*</span> <span class="o">(</span><span class="n">pair</span><span class="o">.</span><span class="na">f1</span> <span class="o">-</span> <span class="n">rightMean</span><span class="o">)</span> <span class="o">/</span> <span class="n">count</span><span class="o">;</span>
        <span class="o">}</span>

        <span class="k">for</span> <span class="o">(</span><span class="n">Tuple2</span><span class="o">&lt;</span><span class="n">Integer</span><span class="o">,</span> <span class="n">Integer</span><span class="o">&gt;</span> <span class="n">pair</span> <span class="o">:</span> <span class="n">values</span><span class="o">)</span> <span class="o">{</span>
            <span class="n">leftSd</span> <span class="o">+=</span> <span class="n">Math</span><span class="o">.</span><span class="na">pow</span><span class="o">(</span><span class="n">pair</span><span class="o">.</span><span class="na">f0</span> <span class="o">-</span> <span class="n">leftMean</span><span class="o">,</span> <span class="mi">2</span><span class="o">)</span> <span class="o">/</span> <span class="n">count</span><span class="o">;</span>
            <span class="n">rightSd</span> <span class="o">+=</span> <span class="n">Math</span><span class="o">.</span><span class="na">pow</span><span class="o">(</span><span class="n">pair</span><span class="o">.</span><span class="na">f1</span> <span class="o">-</span> <span class="n">rightMean</span><span class="o">,</span> <span class="mi">2</span><span class="o">)</span> <span class="o">/</span> <span class="n">count</span><span class="o">;</span>
        <span class="o">}</span>
        <span class="n">leftSd</span> <span class="o">=</span> <span class="n">Math</span><span class="o">.</span><span class="na">sqrt</span><span class="o">(</span><span class="n">leftSd</span><span class="o">);</span>
        <span class="n">rightSd</span> <span class="o">=</span> <span class="n">Math</span><span class="o">.</span><span class="na">sqrt</span><span class="o">(</span><span class="n">rightSd</span><span class="o">);</span>

        <span class="n">out</span><span class="o">.</span><span class="na">collect</span><span class="o">(</span><span class="n">cov</span> <span class="o">/</span> <span class="o">(</span><span class="n">leftSd</span> <span class="o">*</span> <span class="n">rightSd</span><span class="o">));</span>
    <span class="o">}</span>
<span class="o">}</span></code></pre></div>

  </div>

</div>

<p><a href="#top">Back to top</a></p>

<h2 id="other-things-to-try">Other things to try</h2>

<p>For a full feature overview please check the <a href="http://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html">Streaming Guide</a>, which describes all the available API features.
You are very welcome to try out our features for different use-cases we are looking forward to your experiences. Feel free to <a href="http://flink.apache.org/community.html#mailing-lists">contact us</a>.</p>

<h2 id="upcoming-for-streaming">Upcoming for streaming</h2>

<p>There are some aspects of Flink Streaming that are subjects to
change by the next release making this application look even nicer.</p>

<p>Stay tuned for later blog posts on how Flink Streaming works
internally, fault tolerance, and performance measurements!</p>

<p><a href="#top">Back to top</a></p>

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